Ellie’s grandmother is German-born, so she’s Ellie’s “Oma”, and in the interest of having Ellie learn German (since picking up multiple languages is easier the younger they are), Ellie wears these German flag stickers when her Oma is looking after her, to remind her to only speak German to her.
The idea is quite simple: Oma speaks German exclusively to Ellie, so the only way that Ellie can communicate with her Oma is by learning German. After a little bit of muddling the two languages up while learning which words belong to which language, babies quite easily and successfully pick up multiple languages. Then Ellie will already have a great skill, that she can practice not only with her Oma, but with her other German-speaking family members. Ja! (My German is limited…)